1. Field of the Invention
The present invention generally relates to plasma processing apparatuses and, more particularly, to a plasma processing apparatus which processes a wafer for producing a semiconductor device by using a plasma generated by a microwave.
2. Description of the Related Art
Recently, in a semiconductor device manufacturing process, plasma processing apparatuses have been used to perform semiconductor producing processes such as a deposition process, an etching process or an ashing or stripping process since high-density integration and fine structure are required for semiconductor devices. Particularly, a microwave plasma processing apparatus has become popular since the microwave plasma processing apparatus is capable of generating stable plasma at a relatively low vacuum of 0.1 millitorr (mTorr) to several tens of mTorr. The microwave plasma processing apparatus generates high-density plasma by using a microwave or a combination of a microwave and a magnetic field generated by a ring-like coil. The above-mentioned plasma processing apparatus is disclosed in Japanese Laid-Open Patents No. 5-343334 and No. 9-181052.
A description will now be given, with reference to FIGS. 1 and 2, of a conventional microwave plasma processing apparatus. FIG. 1 is a schematic cross-sectional view of a conventional microwave plasma processing apparatus. FIG. 2 is a plan view of an antenna member provided in the microwave plasma processing apparatus.
In FIG. 1, the conventional microwave plasma processing apparatus 2 comprises: a process chamber 4; a table 6 provided inside the process chamber 4; an insulating plate 8 defining a top plate of the process chamber 4 and positioned above the table 6; and an antenna member 10 positioned above the insulating plate 8. The process chamber 4 is constructed so that air inside the process chamber 4 can be evacuated to generate a vacuum therein. The insulating plate 8 is formed of a material that can transmit a microwave.
The antenna member 10 has a flat, disk-like shape as shown in FIG. 2, and has a thickness of several millimeters. Above the antenna member 10 is a slow-wave member 16 formed of a dielectric material so as to reduce the wavelength of a microwave in radial directions of the antenna member 10. The antenna member 10 is provided with many slots 14 each of which has an elongated rectangular shape when viewed from a direction perpendicular to the flat surface of the antenna member 10. Generally, the slots 14 are arranged along concentric circles as shown in FIG. 2, or arranged along a spiral.
The plasma processing apparatus 2 has a coaxial waveguide 12 connected to a center portion of the antenna member 10 so as to introduce a microwave generated by a microwave generator (not shown in the figure) into the antenna member 10. The microwave introduced into the center portion of the antenna member 10 propagates through the antenna member 10 in radial directions thereof, and is directed downwardly toward the interior of the process chamber 4. The microwave introduced into the process chamber 4 generates plasma so that a wafer W placed on the table 6 in the process chamber 4 is subjected to predetermined plasma processing such as plasma etching or deposition.
FIG. 3 is an enlarged plan view of the slot 14 formed in the antenna member 10. As shown in FIG. 3, each of the slots 14 has a rectangular shape having a width L1 of between 10 and 20 millimeters and a length L2 of about several tens of millimeters. The inner walls of the slot 14 are perpendicular to the flat surfaces of the antenna member 10.
The coaxial waveguide 12 is connected to the center portion of the antenna member 10 so as to introduce a microwave generated by a microwave generator (not shown in the figure). The microwave introduced into the antenna member 10 propagates through the antenna member 10 in radial directions and is directed downward toward the process chamber 4 by the slots 14 provided in the antenna member 10. The microwave introduced into the process chamber 4 generates plasma so that a wafer W placed on the table 6 in the process chamber 4 is subjected to predetermined plasma processing such as plasma etching or deposition. In this case, the microwave directed downward toward the process chamber 4 by the slots 14 is a linearly polarized wave.
It is known that a microwave causes concentration of an electric field near a sharp angle portion when the microwave propagates in a solid medium. Thus, when the linearly polarized microwave reaches the surface of the wafer W in the process chamber 4, a concentration of an electric field occurs along the contour of the wafer W which increases the density of plasma. This results in unevenness of the plasma density over the entire surface of the wafer W. Particularly, such a problem becomes more serious as the diameter of the wafer is increased from 6 inches to 8 or 12 inches since the magnitude of unevenness of the plasma processing is increased.
Accordingly, there is a case in which the input power supplied to the plasma processing apparatus is limited so that an abnormal discharge does not occur due to the unevenness of the electric field. However, in such a case, there is a problem in that the throughput of the plasma processing is greatly decreased.
"Circularly Polarized Slot Antenna Fed by Coplaner Waveguide", technical paper of the Electronic Information Communication Society, B-11, vol. J80-B-II, No.10, October 1997, pp.871-878 suggests an antenna structure not for the generation of plasma but for communication use. The structure of the antenna suggested in this technical paper has a plurality of slots each having a loop-like form so as to generate a circularly polarized wave by the slots. Each of the slots is formed by a square opening provided with a metal member therein. A slot line path is arranged along one of the outer sides so as to transmit the microwave. However, the antenna was developed for the communication use, and the antenna used for the plasma processing apparatus related to the present invention does not require such a waveguide line path.